spatial and temporal variability of particulate matter in the benthic boundary layer at the n.w....

Progress in Oceanography 42 (1998) 61–76

Spatial and temporal variability of particulatematter in the benthic boundary layer at theN.W. European Continental Margin (Goban

Spur)

L. Thomsena,*, Tj.C.E. van Weeringb

aGEOMAR, Wischhofstr. 1–3, 24148, Kiel, GermanybNIOZ, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands

Abstract

Near bottom water samples and sediments were taken during five cruises to 6 stations for-ming a transect across the N.W. European Continental Margin at Goban Spur. Flow velocityspot measurements in the benthic boundary layer (BBL) always increased from the shelf tothe upper slope (1470 m) from 5 to 9 cm s−1 in spring/summer and from 15 to 37 cm s−1 inautumn/winter. Decreasing values were detected at the lower slope (2000 m) and the lowestvalues of ca. 2 cm s−1 at the continental rise at 4500 m water depth. Long term measurementswith a benthic lander at 1470 m show that currents have a tidal component and reach maximumvelocities up to 20 cm s−1, sufficiently high periodically to resuspend and transport phytode-tritus. During these long-term observations, currents were always weaker in spring/summerthan in autumn/winter. Critical shear velocities of shelf/slope sediments increased with depthfrom 0.5 to 1.7 cm s−1 and major resuspension events and Intermediate Nepheloid Layers(INLs) should occur around 1000 m. Chloroplastic Pigment Equivalents (CPE) ranged from0.0 to 0.21mg dm−3, Particulate Organic Carbon (POC) from 12 to 141mg dm−3 and TotalParticulate Matter (TPM) from 0.2 to 10.0 mg dm−3. Aggregates in the BBL occurred with amedian diameter of 152 to 468mm. Data on suspended particulate matter in the near-bottomwaters showed that hydrodynamic sorting within the particulate organic fraction occurred.Phytodetritus was packaged in relatively large aggregates and contributed little to the totalorganic carbon pool in nearbottom waters (CPE/POC ca.0.2%). The main organic fraction haslow settling velocities and high residence times within the benthic boundary layer. As POCwas not concentrated in the near bed region the degree to which carbon is accessible to thebenthic community depends on aggregate formation, subsequent settling and/or biodeposition

* Corresponding author. Tel:1 49-431-600-2122; fax:1 49-431-600-2928; e-mail: [email protected]

0079-6611/98/$ - see front matter 1998 Elsevier Science Ltd. All rights reserved.PII: S0079 -6611(98)00028-7

62 L. Thomsen, T. van Weering /Progress in Oceanography 42 (1998) 61–76

of the POC. Close to the sea bed downslope transport may dominate. Under flow conditionshigh enough to resuspend fresh phythodetritus from sediments at the productive shelf edge,this could be transported to 1500 m (Goban Spur) or abyssal depth (Canyon site betweenMeriadzek and Goban Spur) within 21 days. 1998 Elsevier Science Ltd. All rights reserved.

1. Introduction

Recent research at different continental margins has indicated that in addition tothe vertical export flux of organic particles, as measured by sediment traps, lateraladvection of particulate matter has to be invoked (Jahnke, Reimers, & Craven, 1990;Biscaye, Flagg, & Falkowski, 1994; Graf, Gerlach, Linke, Queisser, Scheltz,Thomsen, & Witte, 1995; Thomsen, & Graf, 1995). Lateral transport is largely con-fined by the hydrodynamics within the BBL, which is most important for theexchange between sediments and water column (Dickson, & McCave, 1986;Thorpe, & White, 1988); however detailed observations of BBL characteristics atcontinental margins are extremely rare (Sternberg, Johnson, Cacchione, & Drake,1986; Smith, Jumars, & Demaster, 1986; Townsend, Mayer, Dotch, & Spinrad, 1992;Thomsen, & Graf, 1995; Thomsen, Graf, Juterzenka, & Witte, 1995).

The processes that laterally distribute and sort the material settling out of the watercolumn are as important as is vertical settling in determining the availability of thismaterial as energy source for the benthic carbon demand (Graf, 1992; Auffret, Khri-pounoff, & Vangriesheim, 1994).

The aim of this study was to analyse in situ short-term bottom water flow con-ditions, particle quality and quantity, and size, settling velocity and particle residencetime in relationship to hydrodynamic conditions. The study site at the N.W. EuropeanContinental Margin (Goban Spur) was chosen within the framework of the multidis-ciplinary European OMEX I project on ocean margin exchange processes. The gen-eral aim of OMEX I was to study the physical, chemical, geological and biologicalprocesses at ocean margins—the shelf break and the slope—that determine the trans-port of matter from the shelf to the deep sea. The Celtic Sea is connected to theopen North East Atlantic characterized by its pronounced seasonality and intenselyfluctuating midwater particle fluxes (Newton, Lampitt, Jickells, King, & Boutle,1994) by a continental margin of 100 to 200 nm in width. One of the aims of theBenthic sub-project within OMEX was to evaluate the role of biological processesin cycling of particulate organic matter within the benthic boundary layer. Ourhypotheses were that lateral mass fluxes of particulate matter by far exceed the verti-cal fluxes, bypass the vertical transport pathway and that aggregates in the BBL playan important role in these fluxes.

63L. Thomsen, T. van Weering /Progress in Oceanography 42 (1998) 61–76

2. Methods

Water and sediment samples were taken during five cruises to the Goban Spur inOctober 1993, January, May and September 1994 and August 1995 along transectsperpendicular to the continental margin. The main transect extended in a line between49°28.44’ N,11°12.3’ W and 49°02.3’ N, 13°42.2’ W and covered water depthsbetween 274 and 4500 m (Fig. 1).

Bottom water was sampled with the BIOPROBE system (Thomsen, Graf, Mar-tens, & Steen, 1994). The sampler consists of a system which sucks discrete watersamples through nozzles at 5, 10, 20 and 40 cm height above the sea bottom (h.a.b.).Additional sampling at 500 cm h.a.b. was carried out with a CTD attached to aRosette frame and by Niskin bottles. Videopictures of aggregates at 20, 40 and 500cm h.a.b. were recorded with the particle camera. Flow velocity in the BBL wasmeasured with a thermistor flow meter at 30 cm h.a.b. and with particle cameras(Thomsen, & Ritzrau, 1996). Turbidity in the BBL was measured with optical back-

Fig. 1. Location of the benthic stations on the main OMEX transect. Map from BODC, UK.


scatter sensors or a Sea Tech 25 cm transmissometer. A normal deployment of BIO-PROBE involved lowering the instrument system to the sea floor using a singleconductor cable. One buoy (Benthos, 25 kg net buoyancy) was tied to the cable at30 m above the instrument. Flow velocity, light transmission in the BBL, compassdirection (position) and battery capacity of BIOPROBE were monitored online viathe conductor cable. Immediately after touching the sea bed another 20 m of thewire cable was payed out so that the wire was slack enough to prevent disturbanceby ship movements. BIOPROBE typically came to rest with its feet having penetratedthe sediment to a depth of 1–4 cm. A period of 6 minutes was allowed to elapsebefore sampling in order to allow any material resuspended during deployment todrift away with the current. This was checked via online data control of the transmis-someter.

During the R.V. Pelagia cruise in August 1995, two aggregate cameras wereattached to BIOPROBE to determine the sizes of resuspended sediments and thehorizontal aggregate flux ( > 100mm) at 20 and 40 cm h.a.b. (Thomsen, & Ritzrau,1996). Additionally, at 5 m and 50 m h.a.b. aggregates were monitored. During acruise with R.V.Meteor in September 1994 multiple corer samples were taken atstations OMEX A, I, B, II and III (Fig. 1), transported to Kiel under in situ tempera-ture and transferred into a recirculating flume. The erosion resistance of sedimentsand phytodetritus was determined via calculation and analyses of the critical bottomshear velocityu*cr, measured with an acoustic Doppler velocity profiler (ADV, Son-tec, USA). The beginning of grain movement was determined visually with a Wilddissecting operation microscope.

The ‘NIOZ’ long-term BBL lander BOBO was deployed at station OMEX II at1296 m water depth from 26 June1993 to 25 May 1994 and nearby station OMEXII at 1453 m depth from 8 June 1994 to 15 September 1995. BOBO contained 4acoustic current meters at 25, 50, 75 and 100 cm h.a.b. and a SeaCat CT probeto measure near bed long term variability of conductivity and temperature. Lighttransmission was measured by two Sea-Tech 25 cm beam transmissometers mountedat 100 and 200 cm h.a.b.

The POC content of the water was measured with a Heraeus CHN-Analyzeraccording to Bodungen, Wunsch, & Fu¨rderer (1991). Chloroplastic Pigment Equiva-lents (CPE) were analyzed spectralfluorometrically with a Turner Fluorometer andcalculated using the equations of Lorenzen (1967). Total particulate matter (TPM),was determined using the method of Bodungen, Wunsch, & Fu¨rderer (1991). Bac-terial numbers were determined by the Acridine Orange epifluorescence direct coun-ting technique of Hobbie, Daley, & Jasper (1977) using a Zeiss ‘Standard’ fluor-escence microscope. For analyses of both bacteria and aggregate sizes a MacintoshPower PC image analyses system was used according to the method of Thomsen, &Ritzrau (1996). The carbon-conversion factor of 0.4 pg Cmm−3 cell volume(Bjornsen, 1986) was used for bacterial organic carbon (BOC) determinations. Set-tling velocities of fresh phytodetritus and fluff (sediment/phytodetritus mixture) fromthe study site were determined in the cool-lab with a settling cylinder (ø 20 cm) underin situ temperature conditions. The sizes of the disaggregated particles, sampled in


the BBL with BIOPROBE were measured with a Coulter Counter. Statistics wereevaluated using the statistical software ‘Stat View II’ for Macintosh computers.

3. Results

3.1. Spatial and temporal variability in the benthic boundary layer at GobanSpur

Fig. 2 shows mean values plus standard deviations of particle concentrations inthe BBL (5–40 cm h.a.b.) under different flow conditions, sampled at all stationsduring two cruises including standard stations sampled during all cruises. They reflecta spring/summer and an autumn/winter situation and revealed the following trends:

During autumn/winter cruises (October and January) spot measurements at 30 cmh.a.b. of averaged flow velocities varied between 8 cm s−1 and 37 cm s−1 (Fig. 2A,black line). Turbulence intensities measured as standard deviation around mean flowvelocity (Middleton, & Southard, 1984) at z30 were 5–15%. Flow velocity increased

Fig. 2. Spatial and temporal variability of particulate matter in the benthic boundary layer [5–40 cma.b.] at Goban Spur. Data are means plus standard deviation from spring/summer, autumn/winter cruisessampled at the full transect during two cruises including standard stations sampled during all cruises. Blackdots represent winter/autumn cruises, circles represent spring/summer cruises. Large standard deviations ofparticulate matter indicate increasing particle concentrations towards the sea floor.


from the shelf station to OMEX II (1470 m), where highest flow velocities of 35–37 cm s−1 were determined during the two cruises. Between 1470 m and 3600 mthe flow velocity decreased to 8 cm s−1. Replicate measurements during the variouscruises at the shelf station and at OMEX II were in the same order of magnitude.

During spring/summer cruises (May and August/September) flow velocities didnot exceed 10 cm s−1. Again, highest flow velocities of up to 8.56 0.9 cm s−1 werefound at the upper slope with decreasing values at the lower slope and lowest valuesof 1.5 6 0.1 cm s−1 at the continental rise at 4500 m water depth. Light transmissionranged from 81 to 85% /0.25m.

Total particulate matter decreased with depth during summer and autumn (Fig.2B). Despite higher flow velocities in autumn, TPM in the BBL was ca. 50% lowerthan in summer. Highest TPM concentrations occurred at stations of lowest criticalshear velocity (Fig. 2B). The critical bottom shear velocity (u*cr) increased with depthfrom 0.48 to 1.65 cm s−1. Between 1000 and 1470 mu*cr doubled from 0.8 to 1.6cm s−1.

In spring and summer the POC content of the bottom waters varied between 40and 80mg dm−3 and showed maximum values under lowest flow conditions (Fig.2A, Fig. 2C). Besides station OMEX II, chloroplastic pigment equivalent (CPE)concentrations were more or less constant with depth, but maximum values werefound under highest flow conditions (Fig. 2A, D). The POC/TPM ratios increasedwith depth (Fig. 2F) and showed maximum values under lowest flow conditions.The CPE/POC ratios showed two maxima, one on the shelf, the second at stationB (Fig. 2E). The BOC/POC ratio was lower than 1% and showed more or less nodecrease with depth.

Low POC/TPM ratios were found at 1000 m, where highest CPE/POC occurred(Fig. 2E, F). BOC/POC ratio was positively correlated with flow velocity and rangedfrom 2 to 4.6%.

3.2. Long term measurements

The ‘NIOZ’ long-term BBL record measured by the BOBO lander showed thatthe current velocities around OMEX II (1470 m) had a strong semi-diurnal tidalcomponent. Over periods from 2–5 days, dominant currents were directed to theNNE and SSW. The (potential) bottom water temperature (100 cm h.a.b.) showeda maximum variation of 0.70°C during the same period. Both temperature and sal-inity signals had a similar tidal covariation as have the currents, but also containlonger term variability. During summer 1994 the light transmission (100 and 200cm h.a.b.) over the entire period showed only very little variation, indicating theabsence of benthic storms or strong resuspension events of fine particles. The modestcurrents rarely exceeded 15 cm s−1.

Maximum flow velocities, exceeding 15 cm s−1 occurred more frequently in 1994and were directed dominantly down-slope. A change into a somewhat higher near-bed energy regime with flow velocities exceeding occasionally 20 cm s−1 during aconsiderable part of the tidal cycle occurred from the end of August (Fig. 3).Especially in October high currents were mainly directed down slope.


Fig. 3. High near bottom current velocities in August/September 1994, as recorded by the ‘NIOZ’ BOBOlander. Only velocities exceeding 15 cm s−1 are presented.

3.3. Particle size in the BBL

In summer 1995 the median grain size of disaggregated particles (measured byCoulter Counter) in the BBL decreased with depth from 100mm at the shelf stationto 50 mm at 1000 m. Values of 18–10mm were found to decrease with depth from1470 to 4500 m (Table 1B).

On the shelf the median aggregate size (measured by particle cameras) variedbetween 341 (resuspended sediments) and 467mm (500 cm h.a.b.) and the highestnumbers of up to 1152 large particles > 100mm dm−3 were observed. Under lowflow conditions at OMEX I (670 m water depth), largest aggregates with mediandiameters of 310 to 468mm and maximum sizes of up to 1400mm were found. Theaggregate size in the BBL between 20 to 40 cm h.a.b. increased off the bed, with


Table 1Median aggregate size d50 (mm) and numbers of particles per liter (n/l) in the BBL in August 1995.“Resuspended sediments” are the video-recorded median particle sizes of sediment surface particles, whichwere resuspended during the impact of BIOPROBE into the sediments; A: measured with the particlecamera and B: measured with a Coulter Counter

u* cm s−1 resus. sedim. 20 cm a.b.mm 40 cm a.b.mm 500 cm a.b.mmA mm B (in dm−3) (in dm−3) (in dm−3)

OMEX A (0.36) 341 100 365 (1152) 382 (770) 467 (1020)OMEX I (0.13) 310 60 384 (470) 468 (380) 444 (446)OMEX B (0.31) 363 54 414 (282) 303 (278) 422 (188)OMEX II (0.57) 216 18 272 (270) 196 (190) 206 (109)OMEX F (0.53) 301 10 358 (323) 277 (144) 297 (224)OMEX III (0.15) 189 8 152 (248) 175 (250) 326 (244)OMEX E (0.11) 298 10 181 (338) 267 (101) –

470 to 380 n dm−3 (Table 1). Particulate organic carbon, TPM and CPE decreasedinto the observed water column.

A narrow aggregate size spectrum without significant differences in aggregatediameter was observed at 1470 m water depth, where the highest flow velocity wasmeasured (Fig. 3A). The median aggregate size varied between 196 and 272mmand no aggregates larger than 700mm were found. Aggregate numbers variedbetween 109 and 270 n dm−3. The POC, TPM and CPE concentration profiles showeda uniform distribution within the BBL and the median (disaggregated) sediment grainsize (measured by Coulter Counter) was 18mm. The same trend, but with highermedian sizes and abundances was observed under similar flow conditions at stationOMEX F (2000 m).

At OMEX III (3600 m), despite similar flow conditions to 670 m depth, largeaggregates (326mm, 144 dm−3) were only found at 500 cm h.a.b. and the resuspendedsediment surface particles were larger (190mm) than aggregates at 20 cm (152mm)and 40 cm (175mm) h.a.b. POC and TPM showed a uniform distribution above thesea bed, CPE decreased and the median (disaggregated) sediment size was 8mm.Under lowest flow conditions at OMEX E (4500 m water depth), the sediments werecovered with particles of about 300mm in size (disaggregated size d50 5 10 mm).Aggregates transported in the BBL were 181 to 267mm in size.

3.4. Concentration ranges and ratios in the BBL (5–500 cm h.a.b.)

Chloroplastic pigment equivalents contents ranged from 0.0 to 0.21mg dm−3 witha mean value of 0.1mg dm−3. Particulate organic carbon concentrations ranged from12 to 140mg dm−3 with a mean value of 46mg dm−3. Total particulate matter concen-trations varied between 0.2 and 10.0 mg dm−3 with a mean value of 3.6 mg dm−3.CPE/POC (%) ratios ranged from 0.1 to 0.6% with a mean value of 0.2%. CPE/POCwas positively correlated to flow velocity (P 5 0.02) with a linear correlation atflow velocities below 15 cm s−1 (r2 5 0.95). POC/TPM (%) ratio varied between


0.3 and 19% with a mean value of 2.2%. BOC/POC ratios ranged from 0.05 to 4.6%.Through the whole period of investigation 35–65% of the bacteria were particleassociated in the BBL at 5–40 cm h.a.b. whereas > 80% were free living at 5 m h.a.b.

3.5. Vertical particle concentrations

Generally there was a change from increasing concentrations of TPM to the seafloor at the shelf and upper slope stations (Fig. 4, open circles) where sandy sedi-ments dominated, to uniform TPM distributions at the deeper stations (Fig. 4,black dots).

Besides the shelf and upper slope stations, during summer and autumn POC wasuniformly distributed in the BBL.

Chloroplastic pigment equivalents, which are accounted for by phytodetritus,revealed different concentration profiles. Like POC it decreased into the water col-umn at the shelf and upper slope stations but this ‘bottom heavy’ concentrationprofile also occurred at deeper stations when flow velocities did not exceed 15 cms−1. Under high flow conditions of velocities$ 27 cm s−1 (autumn 1993) CPE wasuniformly distributed in the BBL. Lowest CPE values were always found at 5 and50 m h.a.b.

The estimated settling velocities of the phytodetritus (which is associated withchlorophyll) were 0.2–0.6 cm s−1. The experimental-determined settling velocitiesof phytodetritus, determined in the settling cylinder increased from 0.5 cm s−1 forrecently arrived phytodetritus to 1.6 cm s−1 for fluff (sediment/phytodetritus mixture)of high lithogenic content. The critical shear velocity (u*cr) for this material rangedfrom 0.9 to 1.2 cm s−1.

The POC data reveals that most of the organic fraction has lower settling velo-cities, which were in the order of 0.01–0.15 cm s−1. Calculated horizontal fluxes forPOC and BOC, calculated from mean flow velocities in the BBL and mean particle

Fig. 4. Charcteristic concentration profiles of median values of Chlorophyll (CPE), particulate organiccarbon (POC) and total particulate matter (TPM) computed from corresponding sampling heights underdifferent flow conditions (CPE) and from different locations.


concentrations were orders of magnitude higher than the vertical fluxes, at least 25kg POC m−2 y−1 and 25 g BOC m−2 y−1.

4. Discussion

4.1. Near bed flow conditions

Reported current measurements along the N.W. European continental margin(Pingree, & Le Cann, 1989) show that the upper slope is blanketed by a persistentnorthward flowing slope current. Thus, resuspended material will be transportedmainly to the north along the slope but with a minor component mixing offshorealong isopycnal surfaces. Pingree, & Le Cann (1989) also presented slope residualcurrents between La Chapelle and Porcupine Bank. The main feature of the slopecurrent is its persistent northwesterly directed alongslope flow. At the Celtic slopebetween Meriazek Terrace and Goban Spur the near bottom currents (1–6 m h.a.b.)at mid depths are markedly directed downslope, reaching mean speeds of 15 cm s−1.Long-term current meter records of 90–213 days duration from the continental slopewest of Porcupine Bank (29–54 m a.b.) revealed that the flow velocity exceeded20 cm s−1 in 17% of the time at depth between 500 and 800 m (Pingree, & LeCann, 1989).

In the study presented high current velocities at the upper slope areas (see Fig.2) were observed during autumn and winter cruises. During that time, erosion ofsediments and along or downslope transport is expected. During spring and summerspot measurements with BIOPROBE revealed currents of less than 10 cm s−1, thusresuspension is of minor importance at these times and particle accumulation canbe expected. Although the BIOPROBE data only show spot measurements with notidal cycle invoked, the short term data are consistent with long term BBL data fromthe BOBO Lander. However, maximum flow velocities, exceeding 15 cm s−1

occurred more frequently in 1994 and were directed dominantly down-slope withincreasing values exceeding occasionally 20 cm s−1 in late summer and autumn.Calculated bottom shear velocities for the long term velocity profiles for currentsexceeding 15 cm s−1 at 25 cm h.a.b. were in the order of 1.1–1.6 cm s−1; or as longasu* > u*cr(phytodetritus)for a distinct time of a tidal cycle, phytodetritus will be resus-pended and transported as suspended mode in flow direction.

4.2. Particle behaviour in the BBL

The observed concentration ranges in the BBL are in the same order of magnitudeas reported from other European continental margins, like the western Barents Seaand the N.E. Greenland Sea (Thomsen, & Graf, 1995; Thomsen, & Ritzrau, 1996;Ritzrau, & Thomsen, 1997). The TPM was in the same order of magnitude asreported for the deep sea during benthic storms under similar flow conditions (Gross,Williams, & Nowell, 1988). Data on suspended particulate matter in the near-bottomwaters from the OMEX site from all stations showed that hydrodynamic sorting


within the particulate organic fraction, produced concentration profiles, which weresimultaneously ‘top heavy’ for the POC fraction and ‘bottom heavy’ for the phytode-trital (CPE) fraction (Middleton, & Southard, 1984).

The different behaviour of organic matter in the BBL (Figs. 2 and 4), lowCPE/POC ratios, high C/N values and similar POC concentrations throughout thetime of investigation indicate that the POC and CPE were not of the same origin.

4.3. The particle fraction with high residence time at mid and lower slope areas

At the study site POC always was uniformly distributed in the BBL below 670m water depth (Figs. 2 and 4), even under the lowest flow conditions. The sameholds true for TPM. Low POC/TPM values (, 2%) indicate the ‘resuspended’origin of this suspended carbon fraction. Highest POC/TPM ratios under lowest flowconditions indicate that POC is resuspended earlier than the inorganic particles whichcontribute more than 90% of the total suspended matter. The low settling velocitiesof the POC (of 0.01–0.15 cm s−1) result in high residence times within the benthicboundary layer. The low CPE/POC ratios (| 0.2%) suggest that the material wasnot enriched in fresh phytodetritus but by mainly old and refractory organic debris,that has passed many resuspension loops (Fig. 5). Dating of the surface sedimentsby Hall, & McCave (1998) revealed an average age of ca. 3500 years BP for theorganic fraction. As the POC was not concentrated in the near bed region the degreeto what this carbon is accessible to the benthic community depends on aggregateformation and subsequent settling of the POC (Fig. 5). Data from the study siterevealed that large organic aggregates of low lithogenic content occurred with settlingvelocities in the order of 0.024 to 0.19 cm s−1. These organic/inorganic aggregateswould slowly settle down and reach the sediment surface, if they were not disaggre-gated as a result of the increasing shear closer to the sea floor (Fig. 5). The uniform

Fig. 5. Simple model of processes acting at different time scales that control the carbon input to thebenthos at Goban Spur.


POC distribution with aggregates of higher settling velocities can thus be eitherexplained by aggregation/disaggregation processes which prevent increasedaggregate/particle concentrations towards the bottom; or because the amount ofaggregated POC was too small to influence the concentration profile. In the studypresented aggregates occurred in the BBL and the presupposition for aggregate for-mation is given all over the year.

On the upper slope and shelf both the TPM and POC depict increasing concen-tration profiles towards the sea floor. At these sandy sediment stations POC hadhigher settling velocities, which resulted in a bottom heavy concentration profile,showing that it was bound to the easily resuspended sandy TPM pool of high settlingvelocities (Middleton, & Southard, 1984).

As highest TPM concentrations were found at the upper slope and shelf, evenunder low flow conditions, the sediment grain size and critical bottom shear velocitiesappear to be the factors controlling sediment transport.

4.4. The organic particle fraction with low residence times

Chloroplastic pigment equivalents showed uniform concentration profiles onlyunder high flow conditions at the upper slope. Typically the concentration increasedtowards the sea floor (Fig. 4). These data suggest that the phytodetritus was packagedin relatively large aggregates of high settling velocities (0.2–1.6 cm s−1) and contrib-uted little to the total organic carbon pool in the near bottom waters (CPE/POC ca.0.2%). High CPE/POC ratios under highest flow conditions and at stations of lowcritical shear velocities reveal that CPE was bound to the sediments and had alreadybeen accumulated and modified to fluff. The carbon input of fresh phytodetritus atthe site is pulse-like (Fig. 5). Short events like this have not been traced during thewhole period of investigation as the fresh phytodetritus remains suspended for shortperiods of time. In the case of resuspension caused by increasing current velocitiesat the upper slope and shelf, the CPE/POC ratio increased with decreasing POC/TPMratios as lithogenic surface sediments including phytodetritus (fluff) start to resuspendinto the BBL. However, phytodetritus will rapidly resettle under lower flow con-ditions whenu* , u*cr (phytodetritus) and will be deposited fast (Fig. 5). Only duringtimes of enhanced flow conditions as reported for late summer 1994 (Fig. 3) recentlyarrived phytodetritus could stay in suspension and be advected for longer periodsof time.

A result that is difficult to explain is that despite lower flow conditions TPM insummer were higher than during autumn/winter cruises. One reason for this couldbe that the critical bottom shear velocity of the sediments was different at this time.Dade, Davis, Nichols, Nowell, Thistle, Trexler, & White (1990) presented data on3-fold increases of bottom shear velocity resulting from increasing expolymer pro-duction of the bacteria at the sediment surface. Another explanation could be theenhanced activity of the benthic fauna. There is evidence from flume experimentsthat during times of enhanced activity of benthic surface deposit feeders during theoccurrence of fluff, sediments are actively resuspended into the BBL (Thomsen, &Flach, 1997).


4.5. Implication for sediment accumulation and carbon exchange at thiscontinental margin

4.5.1. The long term sediment accumulationThe upper slope areas at Goban Spur with their enhanced flow conditions are

expected to have relatively low sediment accumulation rates. Recently settledmaterial will be exported. This is consistent with the grain size distribution alongthe transect, showing local winnowing at the upper slope stations, and an increasein the finer fractions to the deeper stations (van Weering, Hall, de Stigter, McCave, &Thomsen, 1998). As highest current velocities were evident at the upper slope (Fig.2) during all seasons, major resuspension events and a formation of INLs are thoughtto take place around 1000 m, which is consistent with the results of Hall, & McCave(1998) and van Weering, Hall, de Stigter, McCave, & Thomsen (1998).

At the study site, high aggregate numbers in the BBL indicate that mass transportvia aggregates plays an important role here. At 1470 m light transmission data ofthe long term lander showed no or little variation, indicating the absence of fineparticles. It has to be considered that transmissometers do not measure aggregates.Thus, mass fluxes of large particles were not detected with the instruments.

Despite similar flow conditions at 670 m and 3600 m larger aggregates were foundat the upper slope, whereas at the continental rise, smaller aggregates are transported.The particle camera analyses of the resuspended sediments reveal that the sedimentsurface at the whole margin is covered with large particles which form the basisfor new aggregation in the BBL after resuspension events. Under these conditionsresuspension loops of particles via aggregation/disaggregation will slowly transportsuspended sediments lateral/downslope to the continental rise.

By calculating the rate of deposition, nominally given by the divergence inmaterial flux by:

Dh/Dt 5 D(uhø)/Dx

whereu is the time-averaged drift downslope,h is the nominal layer depth overwhich u and ø are averaged andø is the volumetric fraction of TPM in the BBL,it is possible to estimate geological accumulation rates at different sites at the conti-nental margin, separated by a distance ofDx (Table 2). Using averaged values of

Table 2Calculation of the rate of deposition, nominally given by the divergence in material flux byDh/Dt 5D(uhø)/Dx, whereu is the time-averaged drift downslope,h is the nominal layer depth over whichu andø are averaged andø is the volumetric fraction of TPM in the BBL to estimate geological accumulationrates at different sites at the continental margin, separated by a distance ofDx

sta./depth u(cms−1) TPM (mgdm−3) Dx (km) uhø (cm2 s−1) Dh/Dt (cm s−1)

II 1400 m 20 4.5 II/F 30 0.0017 5.7 10−10

F 2000 m 10 2.9 F/III 25 0.00056 2.2 10−10

III 3600 m 5 2.7 – – –


spring/summer, autumn/winter cruises the depositional rate would be 6 cm ky−1 forsite F at 2000 m and and 2.3 cm ky−1 at site III at 3600 if downslope transportoccurs about 1/3 of the time. The estimated depositional rate for site III is comparableto holocene sedimentation rates for this station (Hall, & McCave, 1998). The ratefor site F is too high, which might be because the averaged drift downslope at siteF of 10 cm s−1 (u* ca. 0.8 cm s−1) is too high for the sedimentation of fine aggregatedparticles withws of up to 0.2 cm s−1 assuming that particles ofws , 0.64 u* arenot deposited (Allen, 1971). These estimations suggest that the bulk of sedimentaccumulation on the lower slope is the result of downslope transport in and depo-sition from the BBL. Progressive resuspension loops within the benthic boundarylayer would result in long-term horizontal fluxes of fine particles including POC. Asbacteria were bound to particles, carbon input of this material to the benthos occursvia aggregate and feacal pellet formation (Thomsen, & Graf, 1995) and accumulationof this carbon can occur via settling and biodeposition (Thomsen, Graf, Juterzenka, &Witte, 1995; Thomsen, & Flach, 1997).

4.5.2. Fast downslope transport of fresh phytodetritusBecause of the high settling velocities of phytodetritus and its pulse-like input,

transport of this material within the BBL is short and the labile part of the organicfraction is mineralized quickly. However at the upper slope, where higher currentsperiodically occur resuspension of fresh phytodetritus might be possible. This mech-anism would be comparable to the fast downslope transport of rebound phytodetritusdescribed by Walsh, & Gardener (1992). The inclination of the continental sloperesults in rapid near bed lateral transport of this heavy material to the deep sea faster.

Results from the BOBO Lander at OMEX II support these ideas. At this mid slopestation at 1470 m bottom shear velocities exceeding 1.1–1.6 cm s−1 occurred insummer 1994 (Fig. 3). By calculating the progressive downslope vector diagrams,from which U(downslope)averaged over a tidal cycle can be evaluated, the followingtransport of fresh phytodetritus withws of 0.5 cm s−1 andu*cr of 0.9 cm s−1 can beestimated: over a period of 7 days progressive downslope transport of this materialoccurred for 42 h. Assuming that phytodetritus was resuspended to a height of 25–50 cm a.b. and kept in suspension during two tidal cycles per day when currentswere directed downslope a distance of 23–30 km can be covered. Using a half lifeof fresh phytodetritus (Chlorophyll a) of 23 days (Graf, Gerlach, Linke, Queisser,Scheltz, Thomsen, & Witte, 1995) 50% of phytodetritus, settled at the highly pro-ductive shelf edge would still reach the 1000–1500 m depth line at Goban Spur orthe continental rise at the Celtic Margin between Meriazek and Goban Spur, wherea steep slope dominates.

5. Conclusions

The N.W. European Continental Margin at Goban Spur is characterized byenhanced flow conditions at mid slope areas, especially during autumn/winter. Resus-pension of sediments is expected around 1000 m, where critical bottom shear velocity


is low and flow velocities are high. At 1470 m recently arrived material will accumu-late temporarily and will be exported during subsequent times of high flow velocities(10–40 cm s−1). At the lower slope low currents (2–5 cm s−1) support the settlementof carbon.

Hydrodynamic sorting of the organic fraction occurs. Arriving phytodetritus willrapidly settle through the benthic boundary layer, has short residence times in theBBL and contributes ca. 0.2% to the organic fraction. In case of enhanced flowconditions rebound fresh phytodetritus can be transported downslope. Most of thePOC has long residence times. Even under low flow conditions the horizontal fluxesof POC are several orders of magnitude higher than the vertical fluxes.

At the continental margin, BBL-aggregates of organic/inorganic particles occur.Aggregation, settling, disaggregation and sediment contact or uplift can all occur asa result of turbulence. Despite the refractory origin of the organic/inorganic aggre-gates the bacteria attached to the particles can account for 0.1–4.6% of the POC.(Lateral fluxes of bacterial carbon are > 25 g m−2 y−1.)

Close to the sea bed downslope transport may dominate. Under flow conditionshigh enough to resuspend fresh phythodetritus from sediments at the productive shelfedge, this could be transported to 1500 m (Goban Spur) or abyssal depth (Canyonsite between Meriazek and Goban Spur) within 21 days.

6. Acknowledgements

The authors wish to thank the crew of R.V.Pelagia, R.V. Meteorand the depart-ment of Marine Geochemistry of the NIOZ institute, Texel, Netherlands for theirhelp. Brian Dade and Gerard Auffret kindly helped to improve the manuscript. Thestudy was funded by the European Union (MAS2–CT93–0069 and MAS3–CT96–0056). It is a publication of GEOMAR and NIOZ.

7. References

Auffret, G.A., Khripounoff, A., & Vangriesheim, A. (1994). Rapid post-bloom resuspension in the North-eastern Atlantic.Deep-Sea Research, 41, 925–939.

Allen, J.R.L. (1971). A theoretical and experimental study of climbing-ripple cross lamination, with afield application to the Uppsoka esker.Geographical Anniversary, 53A, 157–187.

Biscaye, P., Flagg, C.N., & Falkowski, P.G. (1994). The SEEP II: an introduction to hypotheses, resultsand conclusions.Deep-Sea Research, 41, 231–252.

Bjornsen, P.K. (1986). Automatic determination of bacterioplankton biomass by image analysis.AppliedEnvironmental Microbiology, 51, 1199–1204.

Bodungen, B.V., Wunsch, M., & Fu¨rderer, H. (1991). Sampling and analysis of suspended and sinkingparticles in the North Atlantic.Geophysical Monograph, 63, 47–56.

Dade, B., Davis, J.D., Nichols, P.D., Nowell, A.R.M., Thistle, D., Trexler, M.B., & White, D.C. (1990).Effects of bacterial exopolymer adhesion on the entrainment of sand.Geomicrobiological Journal, 8,1–16.

Dickson, R.R., & McCave, I.N. (1986). Nepheloid layers on the continental slope west of PorcupineBank. Deep-Sea Research, 33, 791–818.


Graf, G. (1992). Benthic pelagic coupling: a benthic view.Marine Biological Annual Review, 30, 149–190.Graf, G., Gerlach, S.A., Linke, P., Queisser, W., Scheltz, A., Thomsen, L., & Witte, U. (1995). Benthic–

Pelagic coupling in the Greenland–Norwegian Sea and its effects on the geological record.GeologischeRundschau, 84, 49–58.

Gross, T.F., Williams, A.J., & Nowell, A.R.M. (1988). A deep sea sediment transport storm.Deep-SeaResearch, 331, 518–521.

Hall, I.R., & McCave, I.N. (1998). Glacial-interglacial variation in organic carbon burial on the slope ofthe N.W. European Continental Margin (48°–50°N). Progress in Oceanography, (this issue).

Hobbie, J.E., Daley, R.J., & Jasper, S. (1977). Use of Nucleopore filters for counting bacteria by fluor-escense microscopy.Applied and Environmental Microbiology, 33, 1225–1228.

Jahnke, R.E., Reimers, C.E., & Craven, D.B. (1990). Intensification of recycling of organic matter at theseafloor near ocean margins.Nature, 348, 50–54.

Lorenzen, C.J. (1967). Determination of chlorophyll and phaeopigments: spectrophotometric equations.Limnology and Oceanography, 12, 17–22.

Middleton, G.V., & Southard, J.B. (1984). Mechanics of sediment movement. S.E.P.M. Short courseNumber 3, 2nd Edition, Tulsa, 787 pp.

Newton, P.P., Lampitt, R.S., Jickells, T.D., King, P., & Boutle, C. (1994). Temporal and spatial variabilityof biogenic particle fluxes during the JGOFS northeast Atlantic process studies at 47°N, 20°W. Deep-Sea Research, 41, 1617–1642.

Pingree, R.D., & Le Cann, B. (1989). Celtic and Armorican Slope and Shelf Residual Currents.Progressin Oceanography, 23, 303–339.

Ritzrau, W., & Thomsen, L. (1997). Spatial distribution of particle composition and microbial activity inthe Benthic Boundary Layer (BBL) of the North East Water Polyna.Journal of Marine Systems, 10,415–428.

Smith, C.R., Jumars, P.A., & Demaster, D.J. (1986). In situ studies of megafaunal mounds indicate rapidsediment turnover and community response at the deep sea floor.Nature, 323, 251–253.

Sternberg, R.W., Johnson, R.V., Cacchione, D.A., & Drake, D.E. (1986). An instrument system for moni-toring and sampling suspended sediment in the benthic boundary layer.Marine Geology, 71, 187–199.

Thomsen, L., Graf, G., Juterzenka, K.V., & Witte, U. (1995). An in situ experiment to investigate thedepletion of seston above a suspension feeder field on the continental slope of the western BarentsSea.Marine Ecology–Progress Series, 123, 295–300.

Thomsen, L., Graf, G., Martens, V., & Steen, E. (1994). An instrument for sampling water from thebenthic boundary layer.Continental Shelf Research, 14, 871–882.

Thomsen, L., & Flach, E. (1997). Mesocosm observations of fluxes of particulate matter within the benthicboundary layer.Journal of Sea Research, 37, 67–79.

Thomsen, L., & Graf, G. (1995). Benthic boundary layer characteristics of the continental margin of thewestern Barents Sea.Oceanologia Acta, 17, 597–607.

Thomsen, L., & Ritzrau, W. (1996). Aggregate studies in the benthic boundary layer at a continentalmargin.Journal of Sea Research, 36, 143–146.

Thorpe, R.S., & White, M. (1988). A deep intermediate nepheloid layer.Deep Sea Research, 35,1665–1671.

Townsend, D.W., Mayer, L.M., Dotch, D., & Spinrad, R.W. (1992). Vertical structure and biologicalactivity in the bottom nepheloid layer of the Gulf of Maine.Continental Shelf Research, 12, 367–387.

van Weering, T.C.E., Hall, I.R., Stigter, H. de, McCave, I.N., & Thomsen. L. (1998). Recent sediments,sediment accumulation and carbon burial at Goban Spur, N.W. European Continental Margin (47–50°N). Progress in Oceanography, (this issue).

Walsh, I.D., & Gardener, W.D. (1992). A comparison of aggregate profiles with sediment trap fluxes.Deep Sea Research, 39, 1817–1834.

spatial and temporal variability of particulate matter in the benthic boundary layer at the n.w....

Documents